This will be our base class for implementing various settings that can
be applied to a file.

My initial thought is that we will load these via
ide_file_load_settings_async(). That will use GAsyncInitable to work
through GIOExtensionPoints that implement various backends. The first
to load would be a modeline that is found in the file. If that is not
found, we would then load .editorconfig or similar. If that is not found,
we can possibly just fall back to local settings (GSettings).

I think I'd like to keep the local settings out of libide and instead
in gnome-builder (it can just register the extension point for a gsettings
loader at startup).